Is Firefox set as default? When installed, Opera will try to copy settings from the default browser automatically. Then again if you were using an extension, you'll need to install the same extension in Opera (Opera copies from the browser, not from extensions).

To fill more on detail about Browser vs. Extension topic. The browsers will not sync custom data if it is not available naturally. In your case, if the extensions need to be added to Opera and imported through extension settings. I would assume that the HTML file that is generated is going to be used as import key to your plugins.